1. Field of the Invention
The present invention relates to components used for splitting and/or combining radiation beams, and in particular to such components suitable for use with high-power radiation beams.
2. Description of the Prior Art
In certain industrial and manufacturing applications involving high-power radiation (e.g., radiation having a power of about 100 W or greater), it is important to maintain beam uniformity or otherwise preserve the integrity of the radiation beam wavefront. For example, in laser thermal annealing or LTA (also referred to as “laser thermal processing” or LTP), high-power laser radiation (e.g., 10.6 μm radiation from a CO2 laser) is used to quickly raise the surface temperature of a workpiece (e.g., substrate such as a semiconductor wafer) to selectively change its properties. The temperature rise is sufficiently fast that the average substrate temperature is not substantially changed, and conduction of heat into the substrate body serves to quench the high surface temperature very quickly. An example LTA application involves annealing and/or activating dopants in the source, drain or gate regions of transistors formed in a substrate in order to form integrated devices or circuits. LTA can also be used to form silicide regions in integrated devices or circuits, to lower poly-silicon runner resistances, or to trigger a chemical reaction to either deposit or remove substances from a substrate (or wafer). Example LTA applications are described in U.S. Pat. Nos. 6,300,208, 6,365,476, and 6,531,681.
In performing LTA, it is important that the radiation beam incident on the workpiece (i.e., substrate or wafer) being annealed be uniform over the area selected for processing, or have a consistent and predictable shape, such as a Gaussian shape. To this end, it is important that any optical system components used to transmit, reflect, split or combine the radiation beam do not alter the beam in a manner that introduces unintended non-uniformities. Such non-uniformities can arise, for instance, when an optical component changes shape or develops an index of refraction variation that distorts the radiation beam wavefront or non-uniformly absorbs radiation over the beam aperture.
Unfortunately, there is a dearth of optical components available for manipulating high-power laser radiation. In particular, there appears to be no commercially available polarizer for polarizing a high-power CO2 laser radiation beam in a manner that leaves the wavefront substantially undistorted, e.g., distorted by less than 150 nm.
One prior art method of making a polarizer involves placing a suitable thin-film coating on one or more zinc selenide windows (substrates). The polarizer is then placed at an oblique angle (e.g., 45°) with respect to the radiation beam incident direction. The coating reflects most of one polarization and transmits most of the orthogonal polarization.
Polarizers are useful for independently attenuating the output power of a CO2 laser radiation beam. The output radiation beam from the laser is typically linearly polarized. The polarization direction is easily rotated by a three-mirror (k-mirror) assembly that rotates about the beam axis leaving the propagation direction undisturbed. By changing the beam polarization direction with respect to a fixed polarizer it is possible to split the beam into two, separated linearly polarized components in which the power can be adjusted over a wide range.
Unfortunately, with prior art beamsplitters (which those skilled in the art understand also act as beam combiners) the absorption of the window material and the associated coating(s) is sufficient to produce appreciable heating resulting in high temperatures near the middle of the window. This, in turn, causes the index of refraction of the window to change, and also causes the surface of the window to distort. The result is that the wavefronts of both the reflected and transmitted radiation beams are distorted. The degree of wavefront distortion varies depending on the intensity of the radiation beam and how long it is incident on the beamsplitter. However, the degree of wavefront distortion is generally unacceptably large for most high-power laser radiation applications requiring a highly uniform, polarized, radiation beam.
Accordingly, what is needed is a beamsplitting component suitable for use with a high-power radiation beam that does not appreciably distort the radiation beam wavefront(s).